Whenever I see the 70 AD date mentioned, I never see any lists of evidence, like "we've carbon-dated the oldest manuscript to have been composed some time after 70 AD" or "the oldest written reference to Mark is from around 70 AD" or anything of the sort. In fact, besides a new development with a mummy mask, I've read that all existing manuscripts date to 100 AD or later.
So what evidence goes into the assertion that Mark was written around 70 AD? How can we verify how correct this reconstructed history is?
(That mummy mask seems to actually answer my question if it really is Mark from around 80 AD, but it seems like the reconstructed history of Mark going back that far predates the mask discovery.)
The earliest manuscripts won't provide the evidence you want to date the composition of the text; all it can do is give you a terminus ante. We have tons of texts we know are older than the earliest extant manuscript. In fact sometimes the entire manuscript tradition has been lost and the earliest copy is a printed edition!
The dating of Mark rests on internal evidence. In Mark 13, Jesus discusses the coming destruction of the Temple in Jerusalem. The secular scholar rejects the notion that Jesus really prophesied that destruction, and rather assumes the writer was writing after the destruction and so knew the details. Titus destroyed the Temple in 70 CE, so the composition of the text had to be after that.
Of course that method of dating won't work if you believe in an infallible Bible, but at that point you're already neck deep in miracles, so....
Frankly given the questions over the sourcing of the mummy masks and the lack of access for independent scholars I wouldn't count them as evidence for anything.
The dates on the gospels are heavily debated. 70 AD is towards the earlier end of the scale for Mark.
An upper limit is 144 AD with the Gospel of Marcion which is the point where Marcion's works were declared heretical. The Gospel of Marcion is a modified version of Luke (although people arguing for later dates may claim it was the other way around). Allowing for the time for a heresy to generate its own version of the gospel and for the thing to be declared as heresy pushes us back maybe a decade. Luke and Matthew both draw from Mark. Allowing time for Mark to become widely enough available that two further gospels use it as a source? Another decade perhaps which gets us to 120AD. Towards the later end for Mark but I've seen it suggested.
As for when/how it was referenced by other authors beyond Marcion there is an early document called First Epistle of Clement which uses text that could be taken from the Acts (but may well not be). The Acts appear to share an author with Luke (theological and Linguistic style). I say "early" for 1 Clement as actual dating is uncertain.
Its starts by mentioning “sudden and repeated misfortunes and hindrances which have befallen us” with “us” being the church in Rome. If that is a reference to persecution then the most likely timing of that is thought to be the persecution under Emperor Domitian (some have argued for Nero but that is a rather fringe position since it seems to refer to Christians now in old age who have been Christians since childhood).
Problem is we aren't totally sure the persecution under Emperor Domitian actually happened due to lack of contemporary references.
The First Epistle of Clement doesn't reference or quote any of the gospels (which can be used as an argument for those who want to claim a late date for the gospels) although it does reference and quote a number of letters. Of course Luke and Matthew don't mention Mark so read into that what you will.